Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/50—NMR imaging systems based on the determination of relaxation times, e.g. T1 measurement by IR sequences; T2 measurement by multiple-echo sequences
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves 
  • A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves  involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/4816—NMR imaging of samples with ultrashort relaxation times such as solid samples, e.g. MRI using ultrashort TE [UTE], single point imaging, constant time imaging
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/5602—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by filtering or weighting based on different relaxation times within the sample, e.g. T1 weighting using an inversion pulse

Definitions

• the invention relates to the field of magnetic resonance (MR) imaging. It concerns a method of MR imaging.
• the invention also relates to a MR device and to a computer program to be run on a MR device.
• Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
• the body of the patient to be examined is arranged in a strong, uniform magnetic field (Bo field) whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based.
• the magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as Bi field) of defined frequency (so-called Larmor frequency, or MR frequency).
• RF field electromagnetic alternating field
• Bi field defined frequency
• the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse), so that the magnetization performs a precessional motion about the z-axis.
• the precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle.
• the magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse.
• 90° pulse the spins are deflected from the z axis to the transverse plane (flip angle 90°).
• the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant Ti (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T 2 (spin-spin or transverse relaxation time).
• Ti spin lattice or longitudinal relaxation time
• T 2 spin-spin or transverse relaxation time
• the decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneity) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing).
• the dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
• the signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body.
• the MR signal data obtained via the RF coils corresponds to the spatial frequency domain and is called k-space data.
• the k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation or other appropriate reconstruction algorithms.
• a T 2 -weighted contrast is often required to characterize tissue lesions detected in MR images (for example in myocardial MR imaging), as the tissue, depending of the type of lesion, has a short T 2 relaxation time and thus appears dark in the T 2 -weighted MR images.
• T2-weighted MR images are conventionally acquired using spin echo (SE) or turbo spin echo (TSE) imaging sequences.
• SE spin echo
• TSE turbo spin echo
• TFE magnetization prepared turbo field echo
• T2prep-TFE is known in the art for some special applications, like cardiac/coronary MRI, in which spin echo sequences are less favourable.
• a problem of the known T 2 preparation scheme are interfering signal contributions without T2-weighting.
• T 2 -weighted MR imaging which is essentially free from interfering contributions from MR signals without T 2 weighting.
• a method of MR imaging of an object positioned in the examination volume of a MR device comprises the steps of:
• a second T 2 preparation sequence comprising an excitation RF pulse, one or more refocusing RF pulses, and a tip-up RF pulse, wherein at least one of the RF pulses of the second T 2 preparation sequence has a different phase than the corresponding RF pulse of the first T 2 preparation sequence;
• the phases of the RF pulses of the first and second T 2 preparation sequences influence the phases of the MR signals acquired by the first and second readout sequences respectively, while they leave the interfering signal contributions resulting from increasing longitudinal magnetization unaffected.
• the interfering signal contributions can be eliminated according to the invention by applying the RF pulses of the first and second T 2 preparation sequences with different phases in combination with appropriate superposition of the first and second sets of MR signals in the finally reconstructed MR image.
• the invention proposes to vary the phase of at least one of the
• the different RF phases in the T 2 -preparation sequences gives rise to different RF phases of the acquired magnetic resonance signal in the different read-outs following the T 2 -preparations. This allows to distinguish the magnetic resonance signal from these read-outs so that interferences from non T 2 -weighted components may be eliminated. This can be done in reconstruction.
• steps a) through d) may be repeated a number of times for sampling a given k-space region, before finally reconstructing the MR image in step e) from the acquired MR signal data.
• the excitation RF pulses of the first and second T 2 preparation sequences have different phases, while the further corresponding RF pulses of the first and second T 2 preparation sequences have identical phases. In other words, only the phase of the excitation RF pulse is varied and the phases of the remaining RF pulses of the T 2 preparation sequences are kept constant.
• the excitation RF pulses of the first and second T 2 preparation sequences have opposite phases, which means that the phase difference of the excitation RF pulses of the first and second T 2 preparation sequences is essentially 180°.
• the interfering MR signals can be eliminated simply by subtracting the first and second sets of MR signals to form a set of difference MR signals, from which the MR image is reconstructed.
• a first MR image can be reconstructed from the first set of MR signals and a second MR image can be reconstructed from the second set of MR signals, wherein the first and second MR images are subtracted to form a difference MR image.
• the subtraction of the MR data for eliminating the undesirable signal contributions may be performed either in k-space or in image space.
• the phase of the tip-up RF pulse of the T 2 preparation sequences may be varied. Also possible is a 90° phase shift of one or several of the refocusing RF pulses.
• the first and second readout sequences are gradient echo sequences, preferably TFE (turbo field echo) sequences.
• TFE turbo field echo
• the first and second T 2 preparation sequences are spatially nonselective. This means that no magnetic field gradients are present during radiation of the respective excitation RF pulses, refocusing RF pulses, and tip-up RF pulses of the first and second T 2 preparation sequences. Without the necessity of rapidly switching magnetic field gradients, the method of the invention enables silent operation.
• the method of the invention is particularly well-suited to generate T 2 -weighted MR images by ZTE imaging or similar silent imaging techniques.
• ZTE zero echo time
• a readout gradient is set before excitation of magnetic resonance with a high-bandwidth and thus short, hard excitation RF pulse.
• FID free induction decay
• the first and second readout sequences of the invention may thus be zero echo time sequences, each comprising:
• the method of the invention described thus far can be carried out by means of a MR device including at least one main magnet coil for generating a uniform steady magnetic field within an examination volume, a number of gradient coils for generating switched magnetic field gradients in different spatial directions within the examination volume, at least one RF coil for generating RF pulses within the examination volume and/or for receiving MR signals from a body of a patient positioned in the examination volume, a control unit for controlling the temporal succession of RF pulses and switched magnetic field gradients, and a reconstruction unit.
• the method of the invention is preferably implemented by a corresponding programming of the reconstruction unit and/or the control unit of the MR device.
• the method of the invention can be advantageously carried out in most MR devices in clinical use at present. To this end it is merely necessary to utilize a computer program by which the MR device is controlled such that it performs the above-explained method steps of the invention.
• the computer program may be present either on a data carrier or be present in a data network so as to be downloaded for installation in the control unit of the MR device.
• Figure 1 schematically shows a MR device for carrying out the method of the invention
• Figure 2 shows a diagram illustrating the T 2 -weighted MR imaging procedure of the invention.
• a MR device 1 which can be used for carrying out the method of the invention is shown.
• the device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field Bo is created along a z-axis through an examination volume.
• the device further comprises a set of (1 st , 2 nd , and - where applicable - 3 rd order) shimming coils 2', wherein the current flow through the individual shimming coils of the set 2' is controllable for the purpose of minimizing Bo deviations within the examination volume.
• a magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.
• a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the
• a digital RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send-/receive switch 8, to a -body RF coil 9 to transmit RF pulses into the examination volume.
• a typical MR imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance.
• the RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume.
• the MR signals are also picked up by the body RF coil 9.
• a set of local array RF coils 1 1 , 12, 13 are placed contiguous to the region selected for imaging.
• the array coils 1 1 , 12, 13 can be used to receive MR signals induced by body-coil RF transmissions.
• the resultant MR signals are picked up by the body RF coil 9 and/or by the array RF coils 1 1 , 12, 13 and demodulated by a receiver 14 preferably including a preamplifier (not shown).
• the receiver 14 is connected to the RF coils 9, 1 1 , 12 and 13 via send- /receive switch 8.
• a host computer 15 controls the current flow through the shimming coils 2' as well as the gradient pulse amplifier 3 and the transmitter 7 to generate imaging sequences according to the invention.
• the receiver 14 receives a plurality of MR data lines in rapid succession following each RF excitation pulse.
• a data acquisition system 16 performs analog-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.
• the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies an appropriate reconstruction algorithm.
• the image is then stored in an image memory where it may be accessed for converting projections or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a human-readable display of the resultant MR image.
• Figure 2 shows a diagram illustrating the imaging procedure of the invention.
• the method starts with a first T 2 preparation sequence T2PREP1 comprising an excitation RF pulse 21, two refocusing RF pulses 22, and a tip-up RF pulse 23. Thereafter, a first readout sequence ROl is applied, which is a ZTE sequence.
• a readout gradient (not depicted) is set before radiation of a short, hard, small flip-angle excitation RF pulse.
• the acquisition of a free induction decay (FID) signal starts immediately after radiation of this excitation RF pulse.
• the next readout gradient is set before the next hard excitation RF pulse is applied and so forth.
• the readout direction is incrementally varied from repetition to repetition until a spherical volume in k-space is sampled to the required extent.
• the FID signals acquired during the first readout sequence ROl form a first set of MR signals.
• This first set of MR signal includes a T 2 -weighted signal contribution 24 and an interfering signal contribution 25 resulting from increasing longitudinal magnetization during MR signal acquisition.
• a second T 2 preparation sequence T2PREP2 is applied which comprises an excitation RF pulse 21 '.
• the excitation RF pulses 21 and 21 ' have opposite phases (i.e. a phase difference of 180°).
• the second T 2 preparation sequence T2PREP2 uses refocusing RF pulses 22' and a tip-up RF pulse 23' having the same phases like the corresponding RF pulses of the first T 2 preparation sequence T2PREP1.
• a second set of MR signals is acquired comprising a T 2 -weighted component 24' and an interfering component 25' resulting from increasing longitudinal magnetization as well.
• the first and second sets of MR signals are acquired with identical readout directions.
• the T 2 -weighted MR signal components 24 and 24' have opposite signs, while the sign of the interfering MR signal contributions 25, 25' is the same in both acquisitions ROl, R02.
• the curves 24, 24', 25, 25' schematically illustrate the amplitude of the respective MR signal contributions as a function of time t during the first and second readout sequences ROl, R02.
• the interfering MR signal contributions 25, 25' are eliminated by subtracting the first and second sets of MR signals to form a set of difference MR signals, from which a MR image is finally reconstructed.
• the final MR image is thus entirely T 2 -weighted without any contribution from non-T 2 -weighted MR signal components.

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• 2016
  • 2016-04-15 US US15/567,388 patent/US10732242B2/en active Active
  • 2016-04-15 EP EP16716566.1A patent/EP3300519A1/en not_active Withdrawn
  • 2016-04-15 JP JP2017554586A patent/JP6684824B2/ja not_active Expired - Fee Related
  • 2016-04-15 WO PCT/EP2016/058303 patent/WO2016169840A1/en active Application Filing
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